This invention relates to circuit breakers, and, more particularly, to circuit breakers having a rotary contact arm arrangement.
U.S. Pat. No. 4,616,198 entitled "Contact Arrangement for a Current Limiting Circuit Breaker" describes the early use of a first and second pair of circuit breaker contacts arranged in series to substantially reduce the amount of current let-through upon the occurrence of an overcurrent condition.
When the contact pairs are arranged upon one movable contact arm such as described within U.S. Pat. No. 4,910,485 entitled "Multiple Circuit Breaker with Double Break Rotary Contact", some means must be provided to insure that the opposing contact pairs exhibit the same contact pressure to reduce contact wear and erosion.
One arrangement for providing uniform contact wear is described within U.S. Pat. 4,649,247 entitled "Contact Assembly for Low-voltage Circuit Breakers with a Two-Arm Contact Lever". This arrangement includes an elongate slot formed perpendicular to the contact travel to provide uniform contact closure force on both pairs of contacts.
State of the art circuit breakers employing a rotary contact arrangement employ a rotor assembly and pair of powerful expansion springs to maintain contact between the rotor assembly and the rotary contact arm as well as to maintain good electrical connection between the contacts. The added compression forces provided by the powerful expansion springs must be overcome when the contacts become separated by the so-called "popping force" of magnetic repulsion that occurs upon over-current conditions to momentarily separate the circuit breaker contacts within the protected circuit before the circuit breaker operating mechanism has time to respond. The thickness of the moveable contact arm as well as the size of the contact springs has heretofore been increased to proportionately increase the overcurrent level at which the popping force causes the contacts to become separated. However, increased thickness and size decreases contact arm mobility and increases the cost.